Ink jet printing apparatus with variable character size

ABSTRACT

A printed dot size is varied by varying a frequency of pressure oscillation applied by a vibratory (18) to ink in an ink ejection head (17), the vibration causing the ink to be ejected from the head (17) and separate into droplets. The variation in ink drop size enables variation of the printed image size by varying an amount of electrostatic deflection of the ink jet and a carriage speed by corresponding amounts.

BACKGROUND OF THE INVENTION

The present invention relates to an ink jet printing apparatus. In suchan apparatus, ink is supplied into an ink jet head and subjected topressure oscillation by a vibrator. This pressure oscillation causes theink to be ejected from the head and separate into droplets. Where it isdesired to print a drop on a sheet of paper, an electrostatic charge isapplied to the droplets. The charged droplets are electrostaticallydeflected by deflection electrodes and hit the paper to print the dot.Where it is not desired to print a dot, the droplets are not charged andare therefore not deflected by the deflection electrodes and hit agutter instead of the paper.

In many applications, it is desirable to be able to vary the size of thecharacters or other image being printed by the apparatus. This has beendone in the prior art by providing a character generator which storescharacter patterns of the various sizes. However, this expedient is notentirely satisfactory due to the requirement of a large charactergenerator for storing the multiple patterns and the control circuitryfor changing between the various sizes.

SUMMARY OF THE INVENTION

An ink jet printing apparatus embodying the present invention includesan ink jet head, vibrator means for applying pressure oscillation to inkin the ink jet head thereby causing the ink to be ejected from the inkjet head and separated into droplets, charging means for charging theink droplets, deflection means for deflecting the charged ink dropletsand carriage means for producing relative movement between the ink jethead and a sheet against which the ink is ejected for printing, and ischaracterized by comprising control means for controlling a size ofimages printed by the apparatus, the control means varying a frequencyof pressure oscillation of the vibrator means and thereby a printed dotsize in accordance with a desired image size and controlling an amountof deflection of the ink droplets and a speed of movement of thecarriage means in accordance with the desired image size.

In accordance with the present invention, a printed dot size is variedby varying a frequency of pressure oscillation applied by a vibrator toink in an ink ejection head, the vibration causing the ink to be ejectedfrom the head and separate into droplets. The variation in drop sizeenables variation in the printed image size by varying an amount ofelectrostatic deflection of the ink jet and a carriage speed bycorresponding amounts.

It is an object of the present invention to provide an improved ink jetprinting apparatus comprising means for varying a printed image size ina simplified manner using a reduced number of component parts comparedto the prior art.

It is another object of the present invention to provide a generallyimproved ink jet printing apparatus.

Other objects, together with the foregoing, are attained in theembodiments described in the following description and illustrated inthe accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram illustrating the basic components of an inkjet printing apparatus to which the present invention applies; and

FIG. 2 is a block diagram illustrating size control means in accordancewith the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the ink jet printing apparatus of the present invention issusceptible of numerous physical embodiments, depending upon theenvironment and requirements of use, substantial numbers of the hereinshown and described embodiments have been made, tested and used, and allhave performed in an eminently satisfactory manner.

Referring now to FIG. 1 of the drawing, an ink jet printing apparatusembodying the present invention is generally designated by the referencenumeral 11 and comprises a pump 12 which supplies ink into an inkejection head 17 through an accumulator 13, three way valve 14 andfilter 16. Ink is ejected from the head 17 by means of an electrostaticvibrator 18 (see FIG. 2) which applies pressure oscillation to the inkat a predetermined frequency. The ink jet separates into droplets in thevicinity of charging electrodes 19 which apply an electrostatic chargeto the ink jet where it is desired to print a dot on a sheet of paper21. The charged droplets are deflected by deflection electrodes 22 andhit the paper 21. Where it is desired not to print a dot, the ink jet isnot charged by the electrodes 19 and is therefore not deflected by theelectrodes 22. In this case, the ink droplets are caught in a gutter 23and do not hit the paper 21. The head 17, electrodes 19 and 22 andgutter 23 are moved relative to the paper 21 perpendicular to the planeof the drawing by a carriage 24 for printing a line. The paper 21 ismoved incrementally in the vertical direction as viewed in the drawingfor line spacing. The ink droplets caught in the gutter 23 are fedthrough a filter 25 to an ink reservoir 30.

Referring to FIG. 2, an essential part of the ink jet printing apparatus11 according to the present invention is shown in block diagram form.The apparatus 11 includes a drive circuit 26 adapted to drive theultrasonic vibrator 18 for the ink jet head 17, a deflection voltagecontrol circuit 27, a carriage drive circuit 28 and a carriage drivemotor 29.

In operation, a switch or the like (not shown) is manipulated to set thedesired dimensions or size of characters to be printed. Then thepressure oscillation frequency of the drive circuit 26 and therefore thedrive frequency of the vibrator 18 are varied. The vibrator 18 in turnvaries the timing at which the jet of ink ejected from the ink jet heat17 separates into droplets, thereby changing the diameter of the inkdroplets. Supposing that ink droplets are of a diameter d mm and spacedλ mm from each other, the ink jet can be stably separated into dropletswhen the ratio λ/d ranges from 4 to 8. Since the dot size on the paper21 formed by impinged ink droplets is proportional to the volume of theink droplets, the dot size or diameter on the paper 21 can be freelyvaried by up to about a factor of two by varying the drive frequency ofthe vibrator 18 and thereby the droplet diameter. Accordingly, thecharacter size on the paper 21 can be varied by altering the deflectionvoltage and carriage feed speed in accordance with the selected size ofthe ink droplets. Where the selected droplet diameter is relativelylarge, the deflection voltage will be made high and the carriage 24 willbe fed at a high rate. For instance, a twice enlarged printing size isobtainable by halving the vibrator drive frequency and multiplying thecarriage speed by √2/2.

The reference numeral 31 denotes a charge data generation circuit and 32a charging signal amplifier circuit. These circuit components 31 and 32,instead of varying the deflection voltage according to a selecteddroplet diameter, may be controlled in accordance with the drivefrequency to adjust the voltage which is applied to the chargingelectrodes 19. If desired, both the deflection voltage and chargingvoltage may be varied.

The circuitry of FIG. 2 further includes an RS flip-flop 33, a charactergenerator 34, a shift register 36 and a switching circuit 37. Theflip-flop 33 is controlled by a print start clock pulse from the drivecircuit 26 and in turn operates the shift register 36 with its Q output.Then, as in a prior art ink jet printer, the switching circuit 37selects a voltage to be applied to the charging electrodes 19.

In summary, an ink jet printer according to the present invention canprint a desired size of characters on a paper sheet by varying theoscillation frequency of the ink jet head. This needs only one set ofcharacter codes in a character generator and therefore cuts down theoverall cost of the printer.

Various modifications will become possible for those skilled in the artafter receiving the teachings of the present disclosure withoutdeparting from the scope thereof.

What is claimed is:
 1. An ink jet printing apparatus including an inkjet head, vibrator means for applying pressure oscillation to ink in theink jet head thereby causing the ink to be ejected from the ink jet headand separated into droplets, charging means for charging the inkdroplets, deflection means for deflecting the charged ink droplets andcarriage means for producing relative movement between the ink jet headand a sheet against which the ink is ejected for printing, characterizedby comprising:control means for controlling a size of images printed bythe printing apparatus, the control means varying a frequency ofpressure oscillation of the vibrator means and thereby a printed dotsize in accordance with a desired image size and controlling an amountof deflection of the ink droplets and a speed of movement of thecarriage means in accordance with the desired image size.
 2. Anapparatus as in claim 1, in which the control means is constructed todecrease the frequency of pressure oscillation, increase the amount ofdeflection and increase the speed of the carriage means in order toincrease the printed dot size.
 3. An apparatus as in claim 1, in whichthe control means is constructed to increase the amount of deflection byincreasing a voltage applied to the deflection means.
 4. An apparatus asin claim 1, in which the control means is constructed to increase thedeflection by increasing a voltage applied to the charging means.